Manufacture of mirrors



Patented Apr. 27, 1943 MANUFACTUREOF masons Elmer J. Ballintine,Tarentum, Pa., assignor to Pittsburgh Plate Glass Company, AlleghenyCounty, Pa., a corporation of Pennsylvania I No Drawing. ApplicationApril 26, 1941,

Serial No. 390,585

' 4 Claims.

The present invention relates to the manufacture of mirrors and moreparticularly to a process of silv'ering glass sheets.

One object of the inventionis the provision of a process in whichmetallic silver deposited from a silvering solution may be confined to asingle surface of a glass sheet-.-

A second object of the invention is the provision of a process in whichthe metallic silver deposited'from' a silveringsolution may be'prevented from adhering to the face of a. glass sheet undergoingthe'silvering operation.

Other objects and advantages of the invention will becomemore apparentfrom the following detailed description of certain preferred embodimentsthereof.

Mirrors are ordinarily prepared by pouring a silvering solution over theupper surface of a glass sheet maintained substantially in a horizontalposition. This process has been followed, with only slight variation,for many years. An excess of the silvering solution is employed,however, to insure complete coverage of the glass surface and thedeposition of a silver film of adequate thickness. This excess ofsolution flows over the edges of the sheet and is drawn onto the undersurface of the sheet where some silver is also deposited. Removal of thesilver partially covering what is to be the face of the mirror isessential and this entails an additional operation which increasesproduction costs. Furthermore there is a danger of permanently impairingthe glass surface and destroying the reflecting film on the mirroredsurface.

Various attempts have been made to prevent the silvering solution fromflowing onto the under glass surface, but these have not been uniformlysuccessful. For example, it has been proposed to form a rim of rubber orputty about the margins of the glass sheet in order to confine thesilvering solutions to a single surface thereof. Obviously these rimsmust be fitted by hand to sheets of varying sizes. A provision of thistype can be used only in an intermittent process and is not adaptable toa continuous line operation in which the glass sheets are handled on aconveyor.- Other attempts to protect the mirror face embody theapplication thereto of paint and resin coatings. These will prevent thedeposition of silver but the removal of the protective coating is asdifficult and laborious as the removal of the silver itself.

Briefly stated, the present invention contemplates the application of anacidulated solution of a dichromate or a permanganate to the surare thenplaced in a horizontal position upon a will form satisfactory treatingsolutions.

table or conveyor system. The upper surfaces of the sheets aresensitized by treatment with a 10.

solution of stannous chloride prior 'to the application of a silveringsolution comprising a mixture of a silver salt and a reducing agent fromwhich metallic silver is deposited on the glass. During the silveringoperation, the excess of the silvering solution flows over the edges ofthe sheets and is. drawn back onto the under surface of the sheet wheremetallic silver is also deposited.

I have discovered that by applying an acidulated solution of adichromate or a permanganate to the under surface of the glass, prior tothe silvering operation, it is possible to prevent any silver depositedfrom the silvering solution from adhering to the treated surface. It isbelieved that the coating forms a masking film over the glass surface tothe extent that no silver nuclei attach to the glass. It is possible,however, that the coating alters directly the chemical nature ofthesiiver reduced from solution for boththe dichromates andpermanganates act as strong oxidizing agents. The coating does free theglass surface of the objectionable silver films which are ordinarilyformed thereon.

KaCrzO'i, NaLzCrzOv and (NHQzCraOv in 'concentrations of from 0.1 to 5percent by weight in an aqueous solution "acidulated with from 1 to 10percent by weight of nitric or sulfuric acid Optimum protection has beenobtained by using a solution containing approximately 5 percent byweight of KzCrzO-z and 1 percent by weight of sulfuric acid.

N aMnO4 and KMnOr-in similar concentrations in 1itcidulated solutionsmay also be used successfu y.

The solution may be sprayed, rolled or otherwise applied to the surfaceof the glass which is to be protected. The coating remains effectiveeven after it is dry and glass sheets may be treated for some time priorto being silvered. When the protection is no longer required, the

coating is easily removed by washing and the glass surface is restoredto its original condition.

It will be understood that various modifications in the concentrationsof the solutions and their manner of application are possible withoutdeparting from the spirit of. the invention or the scope; of theappended claims.

What. I claim is:

1; In the process of manufacturing mirrors which includes the.depositionv of metallic silver from a solution of a silver salt and areducing agent uponv the upper surface of a glass sheet maintainedsubstantially in a horizontal position, the steps which compriseapplying: an acidulated solution of a compound selected from the groupconsisting of K2C1207, NazCnOv, (NHOzCrzOn NaMnOa and KMnO; to the undersurface of the glass sheet to prevent adherence thereon of silver fromthe silvering. solution overflowing the upper surface and contacting theunder surface of the glass, depositing the metallic silver upon theupper surfacewhile the under surface is covered with the compound, thenwashing away the compound.

2. In the process of manufacturing mirrors which includes the depositionof metallic silver from a solution of a silver salt and a reducing agentupon the upper surface of a. glass sheet.

maintained substantially in a horizontal position .the steps whichcomprise app ying an aqueous 3. In the process of manufacturing mirrorswhich includes the deposition oi metallic silver from a solution of asilver salt; and a reducing. agent upon the upper surface of a glasssheet maintained substantially in a horizontal position,.

the steps which comprise applying an aqueous solution containing from0.1 to 5' percent by weight of a compound selected from the groupconsisting of K2CI2O7, NazCrzOw, (NHQzCrzO-z, NaMnO4 and M04 and from 1to 10 percent by weight of sulfuric acid to the, under. surface of theglass sheet to prevent adherence thereon of silver from the silverlngsolution overflowing the upper surface and contacting the undersurfaceof the glass, depositing the metallic silver upon the uppersurface while the under-surface is covered with the compound, thenwashing away the compound.

4. In the process of manufacturing mirrors which includes the depositionof metallic silver from a solution of a silver salt and a reducing agentupon the upper surface of a glass sheet maintained substantially in ahorizontal. position, the steps which compriseapplying' an aqueoussolution containing approximately 5 percent by weight of K2CI'207 and 1percent by weight of sulfuric acid to'the under surface of the glasssheet to prevent adherence thereon of silver from the silvering solutionoverflowing the upper surface and contacting the under surface of theglass, depositing the. metallic silver upon the upper surface while theunder surface is covered a with the compound, then washing away thecompound.

ELMER J. BALLINTINE.

